The catalytic conversion of methanol to gasoline boiling-range components is a highly exothermic reaction releasing approximately 750 BTU of heat per pound of methanol. This amount of heat release will result in an adiabatic temperature increase of several hundred degrees C. for pure methanol feed. In an adiabatic catalyst bed reactor, this large temperature increase will result in high zeolite catalyst aging rates, and possibly cause thermal damage to the catalyst. Furthermore, such high temperatures could cause an undesirable product distribution to be obtained. Therefore, it is critical to the conversion of methanol to gasoline products to provide sufficient heat removing or dissipating facilities so that the maximum temperature encountered in any portion of the zeolite catalyst conversion step is below an upper predetermined limit.
The preferred temperature control technique employs a heat dissipating gasiform material in combination with the reactant charge passed to the crystalline zeolite conversion step. The use of light hydrocarbon gases, C.sub.5 and lower boiling material, dilutes the reactant before contacting ZSM-5 zeolite catalyst. By using a proper dilution ratio, the exothermic temperature rise in the ZSM-5 catalyst system is readily controlled within desired practical limits. The light hydrocarbon gases thus employed are easily separated from the higher boiling gasoline boiling components and can be recycled to the reactor inlet as diluent.
Typical MTG processes utilizing the acid ZSM-5 type catalysts are disclosed in U.S. Pat. Nos. 3,998,899, 4,052,479, 4,138,440 and 4,304,951, incorporated herein by reference. The fresh feedstock methanol is usually provided as a crude liquid containing a major amount of methanol, a minor amount of water and trace amounts of hydrogen, carbon oxides, lower hydrocarbons, and dissolved inert gas. Since the conversion reaction takes place in vapor phase at elevated temperature and pressure, a considerable amount of thermal energy must be transferred to the feedstock. In typical prior systems such as described by Lee in "Synergy", Vol. 2, No. 1, pp. 5-11, 1982, published by Mobil Research & Development Corp., the methanol evaporating and superheating steps have required a large heat exchange section and substantial pressurization of fresh feedstock prior to mixing with compressed recycle gas.
When the vapor phase methanol feedstock is contacted with active HZSM-5 catalyst at reaction temperature, e.g. about 355.degree. C. to 370.degree. C., it should contain a significant amount of diluent gas to prevent temperature excursion in the catalyst bed.
Complete failure of recycle gas in a single conversion MTG reactor leads to an adiabatic temperature rise of about 500.degree. C. Due to high catalyst activity, unacceptable temperatures may be reached in the upper parts of the bed within minutes. Catalyst deactivation from the elevated temperatures tends to magnify the temperature excursion in the downstream portions of the bed.
Ratio feed control can be employed to provide mixing of the gaseous reactant and diluent streams; however, failure of such a control system might result in dumping a deleterious amount of methanol vapor into the conversion reactor under potentially dangerous conditions. Although an emergency shut-down system could be employed to cut off the methanol fuel and cool the overhead equipment with standby propane or the like, normal operations would be suspended and capital cost increased. The possibility of an uncontrolled temperature excursion in the MTG reactor leads workers in this field to reduce the likelihood that methanol can reach the active catalyst in the absence of diluent. Improved heat transfer systems and techniques for improving the energy recovery for feedstock conditioning have also been sought.